Fluid-assisted injection molding is known as a method for producing a pipe member.
FIGS. 4 and 5 each schematically show a method for producing a pipe member according to a conventional technology in which the fluid-assisted injection molding is used.
FIG. 4 is a diagram showing a state of a thermoplastic material in a molding die cavity in a thermoplastic material filling process according to the conventional technology, wherein FIG. 4(a) is a longitudinal cross-sectional diagram taken along an axial direction of the thermoplastic material in the die cavity, and FIG. 4(b) a traverse cross-sectional diagram taken along a direction perpendicular to the axis of the thermoplastic material in the molding die cavity. These diagrams show that the molding die cavity is filled with non-foamed molten thermoplastic material. FIG. 4(c) is a graph that schematically shows a viscosity distribution η(r) of the thermoplastic material in the molding die cavity as a function of a radial position r of the molding die cavity. This graph shows that the viscosity η of the thermoplastic material at its outer layer is high because the outer layer is in contact with an inner surface of the molding die and therefore loses its heat, as a result in a temperature decrease, and that the viscosity η at a central part of the thermoplastic material is relatively low because the temperature thereof remains high.
FIG. 5 is a diagram showing the state of the thermoplastic material in the die cavity in a thermoplastic material discharging process according to the conventional technology, wherein FIG. 5(a) is a longitudinal cross-sectional diagram taken along the axial direction of the thermoplastic material in the die cavity, showing a state in which the thermoplastic material, having pressurized fluid introduced thereto and thus having low viscosity (high fluidity) at its central part, is pushed out. FIG. 5(b) is a traverse cross-sectional diagram taken along line p-p of FIG. 5(a), showing that a hollow part is formed in the central part. Eventually the pressurized fluid is introduced over the entire length of the molding die cavity and the hollow part is formed in the central part in all cross sections, thereby producing a pipe member.
The conventional method for producing a pipe member illustrated in FIGS. 4 and 5 uses a difference in viscosity that is caused by a temperature gradient based on the distance between the inner surface of the die and the non-foamed molten thermoplastic material filling the molding die cavity (the difference in viscosity between a highly viscous skin layer in the vicinity of a molding die cavity surface and a low-viscous core layer in the central part of the thermoplastic material), to push out only the thermoplastic material located in the central part of the molding die cavity, in order to produce a pipe member.
This method for producing a pipe member depends on the size of the molded pipe member or the timing at which the pressurized fluid is introduced. In the most cases, the thickness is 15 to 20% of the outer diameter of pipe. It is difficult to reduce the thickness of the pipe member. For this reason, the requirement specifications and the requests for weight reduction and material cost reduction cannot be met.
For example, when replacing a metallic pipe of an automobile with a resin pipe obtained through fluid-assisted injection molding, there is a limit in increasing the outer diameter of the pipe from the viewpoint of the layout of the pipe. In case of the resin pipe in which the thickness of a pipe wall reaches 15 to 20% of the outer diameter of a mold, the inner diameter of the resin pipe becomes small when the outer diameter thereof is set to be same as that of the metallic pipe, increasing the pressure loss. Therefore, a problem in a cooling pipe, for example, is that an expensive large pump with high energy consumption is required.
The conventional method for producing a pipe member completely fills the molding die cavity with the non-foamed molten thermoplastic material first and thereafter pushes out a molten core layer constituting 60 to 70% of the total filling amount to form the hollow part. This generates a large amount of excess thermoplastic material to be pushed out, resulting in an increase in the cost of recycling the thermoplastic material. Another problem is that this method requires a large injection molding machine that has an injection unit with an injection volume larger than the volume of a hollow molded article. This method, therefore, is considered uneconomical.
Japanese Patent Application Publication No. 2002-18911 discloses a method for molding a resin pipe having a bent pipe part, the method using the same principle as the above-described method for producing a pipe member. The method disclosed in this patent publication is a method for molding a resin pipe, which uses an injection step of injecting a molten resin into a main die cavity for forming an exterior surface of a resin pipe and a sliding mold insertion step of slide-inserting a plurality of sliding molds up to certain positions in the cavity, to mold an end part inner circumferential surface of all pipe parts, introduces pressurized fluid to the molten resin in a region including a part of the bent pipe part without the sliding molds through at least one of the plurality of sliding molds having the region there between, and removes excess molten resin from the region, the excess molten resin being pushed out by the introduction of the pressurized fluid through the other sliding molds.
This method is capable of molding, with high accuracy, an end part of each pipe part and molding the resin pipe having the bent pipe part as a component. However, this method, too, is not a technology for making the pipe part as thin as possible, the pipe part being formed at least by the pressurized fluid.
Japanese Patent Application Publication No. 2002-141405 discloses a method for molding a hollow molded article, which has the steps of introducing a first molding material into a molding die, introducing a second molding material having a viscosity different from that of the first molding material into the first molding material, and thereafter introducing gas for forming a hollow part into the second molding material.
According to this method, for example, setting the viscosity of the first molding material to be higher than that of the second molding material and introducing the first and second molding materials in small amounts can increase the width of a gas passage and the diameter in the width direction of the hollow part. In other words, this method can realize the effect of molding a thin hollow mold. However, such configuration in which the different types of molding materials are introduced into the molding die complicates at least an device for forming the hollow part, thus creating a new problem of an equipment cost increase.
In addition, although different in terms of technical field, there exists a technology for producing a lightweight foam sheet or foam member using a chemical or physical foaming agent. The chemical foaming agent decomposes at a predetermined temperature to generate gas. Mixing such chemical foaming agent with a raw resin and heating the mixture at an over the decomposition temperature of the chemical foaming agent can generate gas in the raw resin. Advantages of using this chemical foaming agent are that the gas is generated accurately depending on the decomposition temperature, that the decomposition temperature can be adjusted easily by adding a foaming assistant agent, and that foam with closed cells can be obtained easily. The physical foaming agent, on the other hand, is a low boiling point organic compound such as butane, pentane, or dichlorodifluoromethane and gasifies a synthetic resin, mixed with the low boiling point organic compound, by releasing the synthetic resin to a low pressure area, to create gas bubbles. The use of this physical foaming agent is characterized in having excellent solubility due to the affinity between the low boiling point organic compound and the resin and that a high expansion ratio foam can be obtained easily due to its excellent retention performance.
Moreover, in recent years the technical field for producing a foam member has a technology that uses supercritical fluid as a foaming agent. Japanese Patent Publication No. 4339296, for example, discloses a method for producing a thermoplastic resin foam injection mold that uses supercritical fluid. This method adds carbon dioxide or nitrogen supercritical fluid to a molten thermoplastic resin to obtain a miscible state thereof, reduces the temperature of the molten thermoplastic resin to a predetermined temperature while keeping a critical pressure, injects the molten thermoplastic resin into a molding die by means of an injection device while keeping the pressure, and reduces the pressure inside the molding die to generate foam. This can produce foam that has an excellent surface exterior, integrated non-foamed part on its outer layer, and cells with a fine average cell diameter and uniform average cell density.
On the other hand, “Study on Microcellular Foaming Using Supercritical Fluid in Injection Molding (2)” by Mr. Takehiro Yamada (or T. Yamada), Prof. Yasuhiko Murata (or Y. Murata), and Prof. Hidetoshi Yokoi (or H. Yokoi), the 6th issue (2008) of Reports of Saitama Industrial Technology Center, describes the study on visualization of the inside of a molding die by using a glass insert mold and a super microcellular foaming mechanism using supercritical fluid. According to this report, an outer layer has a skin layer with no cells as viewed in a thickness direction, and a central part of the material has a core layer having a large number of cells.
An experiment similar to the one described above is reported in “Visualization of MuCell (microcellular foam injection molding)” by Mr. Michio Komatus (M. Komatus) and Prof. Masahiro Ohshima (M. Ohshima), Seikei-Kakou Vol. 22 No. 2 (2010). This report describes that “the viscosity of a resin outermost surface part (skin) increases drastically because the heat of the resin is drawn to the surface of the molding die during the process of filling the die with the resin.”